This invention relates to an image forming apparatus in which sheets contained side by side in cassettes are fed along parallel transport paths and images are formed on the respective sheets.
There has been known a printer, or like image forming apparatus in which small sized sheet materials such as envelopes are contained side by side in cassettes, data such as addresses output from a data storage provided in a personal computer are formed on the surface of a photosensitive member as toner images, and the formed toner images are transferred to sheet materials being transported side by side.
In the image forming apparatus of this type, a plurality of sheet sensors are arranged above each row of sheets defined in the cassette containing the single stack of sheets or stacks of sheets arranged side by side so as to detect the presence or absence of the sheets contained therein. These sheet sensors consist essentially of a photointerrupter including a light emitting element and a photodetector, and a lever which is provided rotatably between the two elements and comes into contact with the uppermost one of the corresponding stack of sheets in the cassette due to the weight thereof. These sensors are designed to determine the presence of a sheet in the cassette by detecting the height of the uppermost one of the sheets contained in the cassette.
In the conventional apparatus of this type, when the small sized sheets are fed side by side from the cassette, the feed of the sheets is continued even if one stack of sheets runs out (there is no sheet fed along one of the transport paths) until the other stack of sheets run out.
Thus, a toner image to be transferred to the sheet fed from the stack of sheets that has run out deposits on the transfer device and the transport path downstream therefrom, thereby smearing the same. Further, the smear on the transfer device and the like deposits on a sheet fed next, thereby smearing that sheet.
As a countermeasure for this, it can be considered to set the same number of sheets in each stack. However, this requires the precise counting of the sheets each time the sheets are set, and thus an operator is obliged to take time and labor to set the sheets.